Normally one locks or sets both suspensions to a really hard setting to not loose to much force while going uphill. I'm however having problems on steep passages, as my front wheel lifts off the ground, or if I shift to much weight to the front, the back wheel slips. That happens especially often on rooty paths or loose ground.

I'm riding a 29" with 120mm suspension at the front with three resistance settings. The back suspsension has also got three resistance settings. I can't lower my suspensions that they stay at e.g. 80mm.

Question: Is it smarter to set the front suspension to an low resistance, so that the angle for falling off the back is not as steep? Or does the increased bouncing (rebound) of the suspension just increase the risk of toppling over backwards, e.g. when shifting more weight to the rear to gain traction?

2 Answers
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The reason for locking suspension uphill is efficiency converting the power you produce into forward momentum. If you are losing forward momentum by slipping, its because you have too much power going to the rear wheel. This needs to only be for a very short (instantaneous) burst. Once traction is lost, it takes a lot to regain it. Give this, there is no benefit in locked out suspension under these situations.

So, all else being equal, uphill slippery conditions set suspension as hard as you can without slipping. On tree roots, that is probably as soft as it will go.

Technique is really important. You need to ride smoothly and in control of power. You need to predict when the rear wheel is rolling over the tree root and reduce power till over it. High cadence helps as long as you can maintain smooth power. Use momentum to carry you over the root, power up when you have traction and power down before you loose it. Reduce weight on the wheels as they hit the tree root to allow it to roll over smoothly (critical on a hardtail, but helps with softtails). Ideally don't hit a root with front and back at the same time, pick your path to avoid it.

Lifting front wheel and loss of control is caused by too much torque (power) on the back wheel - the same smooth controlled power delivery to climb roots will prevent this. Depending on your bike setup and riding style, you will eventually get to the point you cannot ride up the hill without lifting the front wheel.

Front suspension should not be pushing up fast enough (rebound) to cause you to lift the wheel. If it is, adjust you rebound (if you have it). If the trail is ride able, you won't topple backwards unless you do something silly and are completely out of control.

In addition to this, getting the right balance of weight over the front/rear wheel will help on the steepest parts.
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Mark WOct 4 '13 at 13:08

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When my rear wheel rolls over obstacles (especially slippery ones) I use to push the bike forwards. That helps with traction loss, and also in dry conditions it minimizes the power you need to overcome the obstacle.
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bigstonesOct 7 '13 at 19:46

Suspension will not change your overall traction. That is determined by your weight and weight distribution, the tire material and the surface you're riding on, and the contact patch size (determined by tire pressure).

What suspension improves is the compliance to an uneven surface for better handling, and of course it improves your comfort. These things only matters when you're going fast, hitting numerous bumps every second.

Okay, sure, if your front fork is compressed by, say, four inches, then the front has to lift by four inches before the tire loses actual contact. But that is academic; you lose traction before you lose contact. As the fork decompresses due to shifting weight, the force exerted by the tire on the ground lessens. The same thing happens with a hard fork too, as you shift the weight toward the rear; the hard for just does not move up and down with varying weight to show you what is happening. The suspension fork essentially serves as a measurement gauge telling you how much you're shifting weight toward the front or rear.

@cherouvim Suspension helps traction by ensuring compliance of the unsprung mass (wheel and anything that moves with it) with the surface. That doesn't seem to be very relevant when you're climbing a steep hill. The frequency of movement is so low that the entire mass can easily comply to the surface. Generalities like "suspension affects traction" are only useful when we can recognize when they are relevant.
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KazDec 11 '13 at 22:04

Suspension does affect traction, even on uphills.
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cherouvimDec 12 '13 at 5:42

@cherouvim How about if I'm sitting on a bike in one place, not moving anywhere. Am I getting more friction between ground and wheel, because the bike has suspension?
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KazDec 12 '13 at 16:31